This invention relates to soft magnetic materials. More particularly, the invention relates to a method in which iron powders are pressed and subsequently heat treated. The method is particularly useful to make magnetic core components having improved soft magnetic properties.
Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are first moulded in a die under high pressures in order to produce the desired shape. After the moulding step, the structural component usually undergoes a sintering step to impart the necessary strength to the component.
Magnetic core components have also been manufactured by such power metallurgical methods, but the iron-based particles used in these methods are generally coated with a circumferential layer of insulating material.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetized or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to a rapidly varying field, the total energy of the core is reduced by the occurrence of hysteresis losses and/or eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions.
Magnetic core components are made from laminated sheet steel, but these components are difficult to manufacture to net shape for small intricate parts and experience large core losses at higher frequencies. Application of these lamination-based cores is also limited by the necessity to carry magnetic flux only in the plane of the sheet. Sintered metal powders have been used to replace the laminated steel as the material for the magnetic core component, but these sintered parts also have high core losses and are restricted primarily to direct current (DC) operations.
Research in the powder metallurgical manufacture of magnetic core components using iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties include low core losses, a high permeability through an extended frequency range, high pressed strength, and suitability for compression moulding techniques.
In order to decrease losses in a core component for AC power applications it is generally required that the particles have an electrically insulating coating or layer which prevents contact between the particles. An important feature of this layer is its ability to withstand high temperatures without degrading as it is generally desired to heat treat compacted bodies prepared from the powders in order to achieve stress relief. Particularly interesting are insulating layers that can resist temperatures above the recrystallisation temperature of the base material, ie the non insulated powder, without degrading the insulation as important changes to the properties of the base powder occur after the recrystallisation. Another requirement is that the coating should influence the density of a compacted product prepared therefrom to smallest possible extent.
Insulating coatings are described in e.g. U.S. Pat. No. 5,798,177 and DE 34 39 397. According to these publications the coatings are obtained by treating iron based powders with coating solutions including phosphoric acid. The compacted product prepared from the insulated powders is subsequently heat treated.
U.S. Pat. No. 3,245,841 discloses the production of a magnetic powder having high electrical resistance. The powder is prepared by treating an iron powder with a coating solution including phosphoric acid and chromic acid. Compressed powder cores prepared from such a powder are suitable for use in high frequency power devices. The patent is silent about the possibilities to heat treat products prepared from the powders.
Another type of coating is disclosed in U.S. Pat. No. 4,602,957. According to this patent a magnetic powder core, suitable for use in a low frequency power device, is prepared by treating an iron powder with an aqueous solution of potassium dichromate, drying the powder, compressing the powder to form a compact and heat treating the compact at substantially 600xc2x0 C.
However to the best of our knowledge the insulated powders prepared as described in these publications do not combine the properties of high thermal resistance with high compressibility.
One object is to provide compressed materials, which are distinguished by good soft magnetic properties, such as low total loss, high permeability, low coercivity, good frequency stability etc.
A second object is to provide soft magnetic materials which can withstand temperatures at and above the recrystallisation temperature of the iron base material without the insulating layer surrounding the iron base particles being degraded.
A third object is to provide a powder, which may be compacted to high densities in a single compacting step.
A fourth object is to provide a soft magnetic material useful for AC power applications at low and moderate frequencies.
According to the present invention the above objects may be obtained by a process comprising the following steps
compacting an iron based powder, the particles of which are insulated by a chromium containing layer at a pressure between 300 and 1500 MPa; and
heating the compacted body to a temperature sufficient for achieving stress relief and recrystallisation of the iron based material.